The present invention relates to a catalyst for decomposition of hydrocarbons, a process for producing the catalyst and a process for producing hydrogen using the catalyst, and more particularly, to a catalyst for decomposition of hydrocarbons in which fine metallic nickel particles as a catalytically active component exist in the vicinity of the surface of particles constituting the catalyst, thereby minimizing the nickel content, and which exhibits an excellent catalytic activity, a process for producing the catalyst, and a process for producing hydrogen using the catalyst.
Hydrogen has now been noticed as a raw material for new energy and applied to fixed bed fuel cells that are expected to come into wide use in home and small- or medium-scale offices, as well as electric power plants. Therefore, “in-situ” production of hydrogen has been required. For example, in the case of fuel cells for domestic use in which hydrogen is produced from city gas, there have been generally used catalysts on which noble metal elements are supported, resulting in very expensive fuel cell systems. In particular, in order to widely spread the fuel cell systems for domestic use, it is inevitably required to develop low-price systems. Meanwhile, a large portion of the price of the whole fuel cell system is occupied by that of the catalyst used therein. That is, in order to promote wide spread of the fuel cell systems for domestic use, it is required to not only develop a higher-performance catalyst but also realize reduction in price thereof. Therefore, it has been strongly demanded to provide catalysts using more inexpensive elements instead of those using expensive noble metal elements.
For example, nickel is one of the more inexpensive metal elements suitable as an alternative material for noble metal elements. As the method for producing the conventional nickel-based catalysts, there are known a method of spraying an aqueous nickel salt solution, etc., to a bead-shaped carrier made mainly of alumina and then heat-treating the sprayed carrier to support metallic nickel onto the surface of the bead-shaped carrier; a method of preparing particles containing aluminum, nickel, etc., by co-precipitation method and then shaping and heat-treating the obtained co-precipitated product; or the like.
Further, in the case where hydrogen is produced by steam-reforming of low-molecular hydrocarbons, the low-molecular hydrocarbons are contacted with a catalyst in the presence of steam, thereby obtaining hydrogen. However, when the catalysts produced by the above-described conventional methods are applied to such a steam-reforming process, the catalysts suffer from such a coking phenomenon that carbon is deposited on the surface of the catalysts, resulting in deterioration in catalytic activity thereof.
The catalyst used upon the production of hydrogen exhibits more excellent properties as the particle size of metallic nickel particles contained therein is reduced. Namely, the nickel-based catalysts which have an excellent anti-coking property and are optimum for production of hydrogen, are produced by reducing the particle size of the metallic nickel particles to not more than 40 nm, especially not more than 10 nm.
However, the conventional nickel-based catalysts used for production of hydrogen contain metallic nickel particles having a particle size as large as several ten nanometers and, therefore, exhibit a poor anti-coking property, so that properties thereof are considerably deteriorated with the passage of time.
Although the use of hydrogen as a raw material for new energy instead of petroleum reduces a burden to global environments, an increased amount of nickel used not only lead to higher catalyst costs, but also is undesirable from the viewpoint of saving of resources. Therefore, it is necessary to provide a catalyst having a nickel content as low as possible. Further, since the use of a large amount of steam is disadvantageous from the standpoint of energy consumption, it has been strongly required to provide a catalyst capable of retaining its catalytic activity even upon use of a small amount of steam.
In the case where the amount of nickel used in the conventional nickel-based catalysts is reduced, the metallic nickel particles tend to be agglomerated upon spraying or heat treatment thereof. Therefore, there tends to be caused such a problem that the catalyst properties, especially its methane conversion percentage, are remarkably deteriorated in proportion to the reduced amount of nickel.
Thus, it has been strongly required to provide a catalyst capable of not only maintaining a fine particle size of the metallic nickel particles but also exhibiting an excellent anti-coking property even under a low steam atmosphere, and maintaining a good methane conversion percentage even at a less nickel content.
Conventionally, there has been proposed the use of a compound composed mainly of magnesium, nickel and aluminum as a catalyst (Japanese Patent Publication (KOKOKU) No. 48-13828(1973), Japanese Patent Application Laid-Open (KOKAI) No. 50-4001(1975), Japanese Patent Application Laid-Open (TOKUHYO) No. 2000-503624, and Japanese Patent Application Laid-Open (KOKAI) Nos. 11-276893(1999) and 2001-246257). Also, it is described that the catalyst including a carrier made of an oxide of magnesium and aluminum, and containing metallic nickel carried thereon in an amount of about 15 to 280% by weight is used as a catalyst for production of hydrogen by steam reforming (F. Basile at al., “JOURNAL OF CATALYSIS”, 173, (1998) pages 247 to 256).
More specifically, in Japanese Patent Publication (KOKOKU) No. 48-13828(1973), Japanese Patent Application Laid-Open (KOKAI) No. 50-4001(1975), Japanese Patent Application Laid-Open (TOKUHYO) No. 2000-503624 and Japanese Patent Application Laid-Open (KOKAI) No. 11-276893(1999), there are described the catalysts containing magnesium, nickel and aluminum. However, since nickel is uniformly distributed over a whole portion of particles constituting these catalysts, a large amount of nickel is contained therein.
Meanwhile, in Japanese Patent Application Laid-Open (KOKAI) No. 11-276893(1999), it is described that the amount of metallic nickel carried on the catalyst is preferably 0.1 to 10% by weight. However, in the case where the amount of nickel carried is small, as described in Examples of Japanese Patent Publication (KOKOKU) No. 48-13828(1973), the methane conversion percentage is less than 75% or less than 48% at a temperature of less than 800° C. and, therefore, is unsatisfactory.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 2001-246257, there is described a catalyst for partial oxidation of methane, which is composed of a composite metal oxide produced by calcining calcium/aluminum-based laminar composite hydroxide particles containing nickel. However, since nickel is uniformly distributed over a whole portion of the laminar composite hydroxide particles, a large amount of nickel is contained therein.
Further, in the above technique proposed by F. Basile et al., the amount of Ni must be increased in order to attain good catalyst properties. As a result, there tend to be caused problems such as exhaustion of resources, expensiveness of resultant fuel cell system due to increase of catalyst unit cost and large amount of nickel used.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that when composite particles obtained by heat-calcining composite hydroxide-type particles composed of composite hydroxide core particles containing magnesium and aluminum and a composite hydroxide layer containing magnesium, nickel and aluminum which is formed on the surface of the respective composite hydroxide core particles, thereby obtaining oxide particles, and then heat-reducing the thus obtained oxide particles to transform a nickel oxide contained in the oxide particles into fine metallic nickel particles, are used as a catalyst for decomposition of hydrocarbons, the catalyst can exhibit an extremely excellent anti-coking property even under a low steam atmosphere upon the production of hydrogen by steam-reforming of low-molecular hydrocarbons composed mainly of methane. The present invention has been attained based on the above finding.